Electromagnetic interference (EMI), which involves the undesired coupling of electromagnetic energy from an electromagnetic energy source to an affected circuit, system, or structure, may be coupled from the source in one of four ways: 1- through conduction along wires; 2- by far field radiation, such as occurs between a transmitting antenna and a receiving antenna separated by approximately one or more wavelengths of the interference frequency; 3- through capacitive (electric field) coupling; and 4- through inductive (magnetic field) coupling.
One example of the EMI problem involves the widespread use of digital computer circuits in automobile control systems. Such circuits are vulnerable to EMI from radio frequency transmitters such as cellular telephones and amateur radio ("ham") transmitters. In order to function properly, the electronic engine and braking system computers within an automobile must be designed to be relatively immune to the influence of the electromagnetic fields generated by such nearby transmitters.
Similarly, telephone instruments, such as ruggedized test telephones, are often used in an environment that includes one or more nearby AM broadcast radio stations having high power radio frequency transmitters and associated antenna systems, and are therefore subject to exposure to electromagnetic energy that may be sufficient to interfere with or completely disable the operation of such an instrument. Field experience of the present inventors has shown that interference to telephones caused by AM radio broadcasts results mainly from radiated RF signals that are coupled into nearby telephones transmission lines and are then conducted as RF currents into telephones connected to the affected transmission lines.
Because most present day telephone designs employ active circuits (amplifiers, automatic gain control circuits, etc.), which provide telephones with certain advantages over the older "passive" designs, they have greater susceptibility to interference caused by unwanted RF currents. This is due to the nature of the active circuitry which has the undesired capability of readily demodulating audio from amplitude modulated RF carriers.
Field experience in the vicinity of AM radio stations has shown that interference to the desired operation of a telephone instrument is manifested in two primary ways: 1- the telephone's electronic circuits demodulate the audio signal component of amplitude modulated RF currents, amplify this unwanted audio signal component, and couple it to the telephone's receiver, thereby making it extremely difficult or effectively impossible to understand intended received speech; and 2- if the RF signal strength is high enough, the internal circuitry of the telephone instrument will be disabled, making it impossible to place a call.
This problem is diagrammatically illustrated in FIG. 1, which shows a circuit equivalent of high powered, commercial, radio broadcast antenna, transmitting RF signals into a two wire, metallic, telephone transmission line located in the vicinity of the antenna transmitter site. The unwanted AM broadcast signal is schematically represented by a noise source 11 having an associated noise source impedance 13. The AM signal is injected through a pair of resistors 15 and 17, that represent a balanced two-wire transmission line 20, to tip and ring connectors 21 and 22 of a telephone set 30. The hardware of telephone set 30 typically includes a printed circuit board 31, housed within an insulating (plastic) housing or case 33. Mounted on printed circuit board 31 are one or more active electronic circuits, which are capacitively coupled to earth by a naturally occurring mutual capacitance CMO. The undesired RF currents are conducted over the two wire (tip and ring) transmission line 20 in common mode fashion.
RF noise tests conducted in the field have revealed that the strength of the RF noise signal, which follows the dotted line path 25 (from the source 11--through tip and ring transmission line 20--the amplifier circuitry of printed circuit board 31--mutual capacitance CMO-to-earth) is high enough to cause interference to the telephone instrument for blocks around the perimeter of an AM radio broadcast facility.
Presently, in the United States, there are no mandatory regulations requiring manufacturers to provide RF immunity in telephone sets. Although the FCC does not regulate RF susceptibility in telephone equipment, it is aware that RFI in telephones is a problem. In 1994, the FCC published the results of an informal field survey on RF interference to telephones in which several field offices tested the susceptibility of various types of telephones at over a hundred locations of reported RFI. In its report, the FCC describes using a "bulletproof" telephone, having a passive design (which is inherently more immune to RF currents than designs employing active circuits, as noted above) that employs LC filtering to reduce RFI.
On the other hand, there are already several European countries that require compliance of telephone equipment with RF immunity specifications. European specifications require that compliance be demonstrated by laboratory testing. There are two consequences of this that will affect the nature of RF filter designs. First, laboratory conditions do not necessarily duplicate the conditions in the field; secondly, specified test levels are considerably lower than those experienced in the field, especially when the telephone is in close proximity to high powered broadcast facilities. Thus solutions to RFI which work in a lab at relatively low test levels will likely be different than those which successfully operate in a practical or `real world` environment.
In addition, in Mexico, Telephonos de Mexico (TelMex) has a requirement that telephones must have RF Immunity. To date the present inventors have not experienced the use of any telephones in Mexico that completely suppresses RFI. Either or both of the fundamental problems described above are typical, AM audio signals are demodulated or the telephone's electronic circuitry is affected (e.g., dialing circuitry disabled by conducted RF current).
Although various shielding/grounding schemes, such as those described in the U.S. Patents to Pesola et al, No. 5,271,056 and Bogese, No. 4,738,638, address the EMI problem in general, neither patent describes the above-referenced problem of unwanted demodulation by the telephone's electronic circuitry of AM broadcast signals as undesired common mode RF currents on the two wire (tip and ring) conductors. The Pesola et al patent describes the use of a ground foil with a raised edge of frame plate for components of a radio telephone. The Bogese patent describes the replacement of one of the conductors of a telephone type modular jack with a ground strap having a wide surface for conducting high frequency EMI signals to ground, or a metallic connector cover provided for the purpose.